Location systems have been widely used for navigation at sea, in the air and on land. Satellite positioning systems, such as Global Positioning System (GPS), are becoming increasingly popular for navigation and route guidance. Furthermore, these systems are being used to support context-related services. Accurate position information can be used in sport to provide value to spectators, athletes and trainers. However, satellite positioning systems, such as GPS, are not ideally suited to such applications, for several reasons.
Firstly, information about position is usually provided at the receiver. However, in context-dependent applications, positional information is usually needed remotely. Furthermore, receivers require good satellite coverage. However, sporting events often take part indoors or in areas without good satellite coverage. Satellite positioning systems are usually accurate to a few meters and can take several seconds or even minutes to determine a position. However, context-dependent applications often require higher accuracy (e.g. to within a few centimeters) and for positions to be available in real-time (e.g. within milliseconds). Lastly, satellite positioning systems provide information about an absolute geographic position. However, in context-dependent applications, the relative positions of objects is often more important. Systems tailored to these specialist applications have been developed. For example, some systems have been optimised for ball tracking. Other systems have been adapted for accurately measuring relative positions of athletes' limbs during training. Others systems have been developed for accurately measuring the time at which the object crosses a line. Many systems operate across a limited area, for example hundreds of meters to a few kilometers, for the purpose of continuously tracking sports players and objects in real-time during events. These systems are generally based on radio location technologies in which the system tracks small electronic tags carried by the target objects or on imaging technologies that use multiple images from cameras to estimate the object's position.
Accurate position information can also be used in asset monitoring and tracking for commercial or business use, such as in manufacturing, mining, farming, retail, event management and others. Many of the positioning systems based on radio location or imaging technologies are well suited to these applications, often working better than satellite navigation systems for reasons similar to those given earlier. WO-A-03016940 describes a system in which a mobile tag transmits a signal, including a pair of tones, to receivers having known positions. The phase difference between the pair of tones received at each receiver is calculated and used as the basis for determining the position of the tag. This system has the advantage that the effective wavelength of the phase measurement is determined by the frequency difference between the pair of tones, but the signal and tones may be transmitted in a convenient radio band. The actual radio band used for transmission is largely unimportant and the relationship between the tones is not affected significantly by radio architectures or system configuration. The system solves many of the problems affecting earlier phase measuring systems, such as the system described in U.S. Pat. No. 5,045,861. However, the system, like many other phase measuring systems, suffers from the problem of multipath. Multipath is difficult to detect and mitigate since the sum of two phase-shifted tones is simply a tone with a different phase.
The present invention seeks to provide apparatus for and a method of determining a position of a tag.